The present invention relates to a heat storage correction apparatus used for a recording device to make thermal records and for a display device using magnetized latent images.
A typical thermal head contains a number of heater elements which are ordinarily aligned to cause the heater elements to generate heat according to certain picture data. The thermal head generatgs a thermal pulse to record picture images in a thermosensitive recording or transfer system and to form magnetized latent images in a display device.
A recording or a display device employing a thermal head makes a record or display (hereinafter just referred to as "record") by using thermal energy. If the energy becomes excessive or insufficient, the density of the picture is adversely affected and the picture quality deteriorates. The risk of deterioration in picture quality becomes larger as the speed of recording the record density increases. It thus becomes necessary to adjust the picture quality to maintain it in good condition.
FIG. 1 shows a picture data arrangement for calculating the heat storage condition of a thermal head. Data row L1 designates the data on a line to be recorded. Data row L2 designates the data in the line previously recorded. Data rows L3-L5 designate respectively the data recorded in the four previous lines.
In data row L1, data D0, shown cross-hatched in FIG. 1, is referred to as "aimed data. " D0 corresponds to the one heater element with respect to which printing processing is presently performed. The printing process refers to the calculation of applied energy to the element. Ten other data, D1-D10, are called "reference data" and are used for calculating the heat storage condition. In the group of reference data D1-D10, for example, D1 and D2, which correspond to heater elements adjacent to the heater element for the aimed data D0, may have relatively great influence on the printing of the aimed data D0. Alternatively, reference data D4, which corresponds to the same heater element on the previous data row L2, may have the greatest influence on the printing of the aimed data D0. Thus, each reference data which influences the heat storage data for printing the aimed data D0 may have different degrees of importance depending, for example, on the distance between heater elements or the printing intervals on each line.
A conventional system weights the respective reference data D1-D10 and adds the weighted data to calculate the heat storage condition. The weighting is as shown in the following Table 1.
TABLE 1 ______________________________________ Reference Data Weight ______________________________________ D1, D2 70 D3, D5 45 D4 160 D6, D8 17 D7 100 D9 60 D10 36 ______________________________________
The thermal energy needed to print the aimed data is determined in accordance with the heat storage data subjected to the weighted addition as described above. This is accomplished by adjusting the time width and the voltage of the pulse applied to the corresponding heater element of the thermal head.
FIG. 2 shows an example of a conversion function between the heat storage data and the applied pulse width in an apparatus in which the applied pulse width is varied to adjust the thermal energy. The abscissa indicates various heat storage data corresponding to sums of the reference data D1-D10 weighted on the basis of the Table 1. Those sums correspond to the heat storage data for the heater element corresponding to the aimed data D0. The sum is zero when all the reference data D1-D10 are non-printing data (white data), and has its maximum value 620 when all the reference data are printing data (black data). The ordinate represents the pulse width in milliseconds (ms).
In FIG. 2, if the heat storage data for the aimed data D0 at a certain point of time is, for example, 620, the applied pulse width is the shortest (0.3 ms) because the element's heat storage condition is greatest. If that condition is zero, the applied pulse width is the longest (0.5 ms).
The applied pulse width is not always determined only on the basis of this weighted sum, but in many practical cases the applied pulse width has been set by referring to the pulse width applied to record the preceding line. However, the basic principle is that the applied pulse width is set shorter as the heat storage data increases.
In using such a heat storage apparatus, however, sometimes the printing density is reduced or foggy when printing a solid black portion or a pattern portion which was substantially solid black (both hereinafter referred to as a "solid portion"). This occurs because the conventional heat storage correction apparatus is normally arranged to suit a printing pattern consisting of lines and/or dots, and too much heat correction is performed for a solid portion.
An object of the present invention is a heat storage correction apparatus for proper printing of even a solid portion.